If you've got a keen eye, you might have noticed the rates displayed on the Buy/Sell pages are slightly different. This difference is called the spread. Swyftx publishes its live spreads compared to other Australian competitors on its website here.
In any market, crypto or otherwise, there will always be a difference in price between what buyers and sellers are willing to trade at. Prices go up when the buyers are prepared to pay the higher prices of the sellers, and go down when sellers are prepared to sell at the lower prices the buys are offering (otherwise known as supply and demand)
In the context of trading, liquidity refers to the number of buyers and sellers that are advertising their prices on the order book. Read more about order books here. Higher liquidity (aka more buyers and sellers) means that Swyftx can quote more accurately and offer better prices for its customers.
When you enter an amount to buy or sell, Swyftx queries the liquidity on partner exchanges (including Binance) to determine how many buyers or sellers it would take to fill your trade. In contrary to the common idea that the more you buy/sell the better price you get, trading on open markets in higher quantities results in less favorable prices (dictated by liquidity of that particular asset). This is known as slippage.
One last thing to note. Swyftx will search its partner exchanges to find those with the best liquidity and best current trading price to fill your orders. This means that prices on Swyftx should always be the best possible, however may be slightly different to prices listed on individual exchanges elsewhere.